I don't see why anyone should have a problem with it, we are all adults on an adult resort. If others don't feel that way, that's fine, no one is asking them to take their top off. There are pleanty of places to sit so they could move to a different area. I know I'd go topless if other were and the staff doesn't say anything! =)
Been to Couples so many times and went on the booze cruise many times. I've seen it girls do it but not many. If she's just lying out and not walking around and dancng and stuff then the staff probably won't mind but I'm sure it depends on who takes you out. I don't think the resort would promote it. She won't want to on the way back bc it does get chilly by then. I hope that people start doing it, would be fun and I would partake for sure! When in Rome right?